$re = '/\(((?:[^()]|\(\))+)\)/m';
$str = 'x (a) y -> a
x (a b) y -> a b
x (a b c) y -> a b c
x (a b() c) y -> a b() c
x (a() b() c) y -> a() b() c
x (a b() c()) y -> a b() c()
x (a b(a) c) y -> a
x (a (b() c)) y -> b() c
x () y -> empty
';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php